1. Field of the Invention
The present invention relates to data transmission utilizing a digital transmission channel that enables the transmission of audio and data services and the selective reception of these services. The information transmitted on the transmission channel can either be in a continuous data stream or packet form.
2. Description of the Related Art
In the digital audio system DAB (Digital Audio Broadcasting), developed for using frequency bands efficiently, the transmission path is completely digital and the system is intended to replace the present generally used analog broadcast system using frequency modulation. The DAB is especially designed for a mobile environment, that is, the receiver can be mobile so that different attenuations and disturbances produced by the propagation of a radio signal can be prevented by the combined effect of suitable modulation and channel coding. A DAB receiver is capable of enduring several echoes originating from the same source and thus a country-wide radio coverage can be achieved with one frequency channel since the signal of stations other than the one to be listened to is interpreted as an echo and will be eliminated.
The DAB determines the digital radio channel, based on many carriers, that is suitable for the. transmission of both audio and data services. A completely digital transmission channel may be either a continuous data stream or packet channel. Packet switching is more flexible and more easily enables the transmission of data units of a finite length. The transmission, transfer and reception of the DAB system is explained in detail in Kimmo Hakkarainen, MSc Thesis, A Channel Encoder/Decoder for DAB Demonstrator, 1995, Tampere University of Technology, pages 2 to 13. For the receiver the highest abstraction level of the DAB system is called an ensemble. One ensemble is divided into several services and each service is further divided into service components. A service component is either an audio channel or a data channel.
FIG. 1 discloses one DAB system in a simplified way. At the transmission end a control device 1 controls the transmission. A FIC and control block 2 produces SI (Service Information) information relating to audio and data services, Multiplex Configuration Information MCI and CA (Conditional Access) information relating to conditional access that can be information relating to chargeability/encryption of the services. Taken together, they form a fast information channel FIC. Audio information, such as music, supplied by producers 3 of audio services is compressed in a MPEG encoder 4 and is led to audio channel encoders 5. Similarly, the data supplied by producers 6 of data services is encoded in data channel encoders 7. The channel encoded data, audio and FIC information are led to a block carrying out orthogonal frequency division multiplexing OFDM. The OFDM symbol produced by an inverse fast Fourier transformation of the block is a group of subcarriers with an exactly determined duration. Individual subcarriers are modulated by a D-QPSK (Differential Quaternary Phase Shift Keying) method and at the end it results in a DAB transmission signal that comprises successive transmission frames. Each frame is time multiplexed between a synchronization channel, a fast information channel FIC and a channel MSC (Main Service Channel) containing audio and data services. Transmission mode I intended for a radio network on the ground using one frequency has 1,536 carriers, a frame has 76 symbols and the duration of a frame is 4* 24 ms=96 ms. Transfers can be made in the frame. Mode II intended for ground-level and satellite transmissions has 384 carriers, a frame has 76 symbols and the duration of a frame is 24 ms. 55,296 MSC bits and 1,024 FIC bits are transferred in a frame. Mode III intended for ground-level, satellite and cable transmissions has 192 carriers, a frame has 153 symbols and the duration of a frame is 24 ms. 55,296 MSC bits and 1,024 FIC bits are transferred in a frame.
The signal received at the transmission end is decoded in a COFDM (Coded Orthogonal Frequency Division Multiplex) block 9 that converts an I-Q signal into a digital mode, the digitized signal is transferred to the frequency level by a fast Fourier transformation, frequency interlacing is removed and the transmission frames are comprised of successive OFDM symbols. The information channel FIC and the channel MSC containing audio and data services are separated from one another and the subchannels are separated from the MSC channel and they are channel decoded in decoders 5' and 6'. The desired subchannels are then conducted to be processed further. The subscriber finds out from the received FIC channel what services the received signal contains and can accordingly select the desired service or services.
One advantage of the DAB system is that data capacity can be reserved dynamically for service providers. For a moment, the capacity can be at most 1,728 Mbit/s. Then data is transmitted in packets, as shown in FIG. 2A, that contain a header field, a data field and a check sum. The meanings of the described fields are in compliance with the DAB standard. The packet header contains information on the length of the packet (PKT LEN) that can be 24, 48, 72 and 96 bytes, a continuity index (CONT IND), the first/last packet information (FIRST/LAST), the address (PKT ADDRESS) identifying the service component, the command, the actual length of the data field (DATA LEN). The data field contains the actual data to be transmitted and stuffing bits when needed. At the end there is a check sum of the packet (PKT CRC).
The data fields of the packets form a so-called Data Group, FIG. 2B. Generally a data group comprises several data fields of packets to be transmitted in sequence. At its simplest, one packet is sufficient for forming one data group.
A data group is comprised as shown in FIG. 3.
The fields of the group header are the following:
______________________________________ EXT FL Extension flag CRC FL CRC flag SES FL Session flags DG TYPE Data group type CONT IND Continuity index REP IND Repetition index EXT FIELD Extension field ______________________________________
The fields of the session header are the following:
______________________________________ LAST FL Last SEG NUM Segment number RFA Reserved LEN IND indicates the length of the next address field ADDR. FIELD End user's address (LEN IND) ______________________________________
These header fields are followed by the actual data and the check sum of the data group.
At least in theory, the DAB system enables the transmission of multimedia and hypermedia type of services to a mobile user. However, a problem is that in the transmission of multimedia and hypermedia services to a mobile receiver no mechanisms have yet been specified for transmitting service components and files so that the software of the receiver would be able to receive, combine, form, present and execute the services in the way desired by the user in accordance with the possibilities produced by the service provider.